Chronological discord between the last interglacial paleosol (S1) and its parent material in the Chinese Loess Plateau

摘要

An and Porter (Geology 25 (1997) 603) reported six high dust-influx events of millennial timescales recovered from the last interglacial paleosol S1 and correlated them to six cool events of millennial timescale in the North Atlantic. However, the complexity of soil-forming processes may have made the chronological correlation with the North Atlantic records inadequate. To examine the complexity of the S1 formation, the S1 paleosol was traced laterally and identified based on the preserved characteristics observed in the field and analyzed in the laboratory. Our data show that from the northwest to the southeast, the S1 paleosol gradually converges from three distinctive soil profiles into a single welded profile because the net rate of loess accumulation was attenuated to the southeast and pedogenic development intensified southeastward during the last interglacial. Three soil-forming events within the S1 paleosol (S1S1, S1S2 and S1S3) separated by two loess units (S1L1 and S1L2) in the northwestern part of the Loess Plateau are stratigraphically coeval with a single soil profile in the southeastern margin of the Loess Plateau. In the southeast, the S1 paleosol developed into underlying older loess L2 (e.g., at the Lantian section). The three paleosols (S1S1, S1S2 and S1 S3) are partially welded in the central part of the Chinese Loess Plateau (e.g., at the Tianshui section), where the lower portion of S1 paleosol developed in the underlying older loess unit L2. In the northwestern margin of the Chinese Loess Plateau (e.g., at the Lanzhou section), the preservation of the repeating soil-loess sequence (S1S1, S1L1, S1S2, S1L2 and S1S3) continuously documented the climatic events of the last interglacial. Our data also show that the magnetic signatures and particle-size information are more or less acceptable climatic proxies only for the northwestern sections, where the degree of pedogenesis was lower and the rate of eolian influx was greater during the last interglacial. It appears that in all cases investigated, the median grain size and the coarse fraction ( > 63 μm) content define the upper and lower boundaries of the S1 paleosol reasonably well and can be used to estimate the time-transgressive nature of the S1 paleosol relative to its parent material. Soil welding, bioturbation and material translocation within the S1 soil profiles make it impossible to preserve the detailed and high-resolution information of climate changes in those S1 profiles in the southeastern part (including the popularly called central part) of the Chinese Loess Plateau.